The present invention generally relates to semiconductor laser control apparatuses, and more particularly to a semiconductor laser control apparatus for controlling a semiconductor laser which is used as a light source in an optical disc apparatus, a laser beam printer and the like.
A control to stabilize an optical output of a semiconductor laser which is used as a light source of an optical disc apparatus, a laser beam printer and the like is essential in order to stably carry out a recording, a reproduction and an erasure of an information with respect to an optical disc on the optical disc apparatus and in order to stably write an information optically with respect to a sheet of paper on the laser beam printer.
Conventionally, a semiconductor laser control apparatus shown in FIG. 1 is used to stabilize the optical output of the semiconductor laser. In FIG. 1, a laser beam emitted from a semiconductor laser 1 is received by a light receiving element 2 and subjected to a photoelectric conversion. An output signal of the light receiving element 2 is amplified in an amplifier 3 and is supplied to a comparator 4. The comparator 4 compares an output voltage of the amplifier 3 with a reference voltage Vref, and signal indicative of a result of the comparison is supplied to an up-down counter 5 as an up-down signal.
The up-down signal becomes an up-signal which instructs an up-count when the output voltage of the amplifier 3 is lower than the reference voltage Vref. On the other hand, the up-down signal becomes an down-signal which instructs a down-count when the output voltage of the amplifier 3 is higher than the reference voltage Vref. The up-down counter 5 counts up or counts down clock pulses of a constant frequency obtained from an oscillator 6 depending on whether the up-signal or the down-signal is obtained from the comparator 4. An output counted value in the up-down counter is converted into an analog signal in a digital-to-analog (D/A) converter 7. The analog signal from the D/A converter 7 is supplied to a semiconductor laser driving circuit 8 which supplies to the semiconductor laser 1 a current proportional to the analog signal. As a result, the optical output of the semiconductor laser 1 is stabilized to a value corresponding to the reference voltage Vref.
However, according to the conventional semiconductor laser control apparatus, the following problems occur when it is used for controlling the light source of the optical disc apparatus, for example. In other words, in a reproducing mode when an information is reproduced from an optical disc (not shown), the up-down counter 5 repeats an up-count and a down-count. For this reason, a ripple is generated in the optical output of the semiconductor laser 1 as shown in FIG. 2 with a frequency which is 1/2 the frequency of the clock pulses supplied to the up-down counter 5 from the oscillator 6.
When an oscillator having an oscillation frequency of several kHz, for example, and avoiding a frequency band of the reproduced signal from the optical disc which is generally in a range of several tens of kHz to several MHz is used as the oscillator 6, there is a problem in that a rising speed with which the optical output of the semiconductor laser 1 rises to a predetermined value when turned ON becomes extremely slow. But on the other hand, when an oscillator having an oscillation frequency of several tens of MHz is used as the oscillator 6, there is a possibility that an even large ripple will be generated in the optical output of the semiconductor laser 1 due to a delay introduced in the amplifier 3.
Another type of semiconductor laser control apparatus using an up-down counter is disclosed in a Japanese Laid-Open Patent Application No. 59-18964, but this other semiconductor laser control apparatus suffers problems similar to those of the conventional semiconductor laser control apparatus shown in FIG. 1.